1. Field of the Invention
The need exists for a reliable non-destructive test for detecting defects in material used for fabricating defect-free objects of high safety and reliability as are required for use in applications such as atomic power and stream power equipments, airplanes, automobiles, chemical plant and machines; iron frame structures in buildings and bridges; and ceramic members.
Ultrasonic flaw detection provides a non-destructive test for detecting defects in materials. An ultrasonic pulse (hereinafter referred to simply as pulse) is emitted from an ultrasonic probe (hereinafter referred to simply as probe) into the object. The pulse is reflected from a defect, if any, and such reflection is detected by the probe thereby to detect the fault in the material. The ultrasonic flaw detection is most generally used for objects of the type mentioned above.
The probe is generally of the ceramic type and it is configured as a piezoelectric transducer. Its characteristics deviate from probe to probe by reason of manufacturing tolerances, and further is frequently exchanged as consumption goods. It is necessary, however, to obtain consistent and reliable quantitative analysis characteristics and a uniformity in test results for similar defects regardless of the probe used. Also needed is an ability to determine a tolerance of the sound pressure distribution of the pulse emitted from each probe. Furthermore, to avoid detection of a fault in actual flaw detection, one must be able to select a suitable probe, an arrangement and a scanning pitch of the probe for the particular application. The most important factors for carrying out the ultrasound fault detection procedure require measurement and assessment of the. pulse waveform emitted from the probe, the sound pressure distribution relative to the waveform, and the changes in their propagation through the object.
The present invention provides a quantitative measuring method for measuring the pulse waveform and the sound pressure distribution on the waveform by emitting from the probe a pulse into a transparent solid model of the object which will be tested. By using photoelasticity, the method enables evaluation of the characteristics concerning the sound pressure distribution of the particular probe, by carrying out tests on a transparent solid model of the material to be tested. The transparent model material is selected to have characteristics which are similar to those of the actually to be tested material. The present invention allows various probes to be characterized and is effective to enable selection of probes suitable for most detecting particular defects, arrangement of probes, determination of a scanning pitch thereof, and a development of new probes. The method serves to improve the reliability and the precision available with ultrasonic flaw detection.
Furthermore, since an ultrasonic pulse produces a weak stress wave in the material, the method of the present invention relates to a technique for measuring weak stresses with sensitivities which are much higher than those available with conventional photoelastic stress measuring techniques. Therefore, the present method can be utilized for static weak stress measurements and for dynamic weak stress measurements of repeat phenomenon.
2. Description of the Prior Art
There are three kinds of methods for measuring pulse waveforms and sound pressure distributions of the pulse emitted from the probe. One of them calls for visually measuring and evaluating the pulse actually launched from the probe into a model material which is similar to the material to be used, in a manner similar to the photoelasticity and the Schilieren methods. Another method uses a standard test piece having minute reflectors in lateral and vertical holes using the actual material to be tested. The last method relies on receiving the pulse emitted from the probe by using an electro-dynamic sensor.
In the above-mentioned methods which use the standard test piece or the electro-dynamic sensor, the sound pressure distribution is consequently evaluated taking account of the characteristics of the probe for receiving the pulse and the electro-dynamic sensor. Therefore, only the relative sound pressure distribution can be measured and information concerning the absolute value of the pulse waveform and the sound pressure can not be obtained. Furthermore, there are disadvantages that the frequency characteristic of the reflection due to the minute reflection body in the standard test piece is very unique, and the reflection waveform is undesirably dependent on the input waveform.
It is assumed that it is effective that the method of visually evaluating the pulse has a possibility of measuring the pulse waveform and the sound pressure on the waveform. However, since the Schilieren method does not provide a visual image which is proportional to the sound pressure, obtaining a quantitative measurement is difficult. In contrast, according to the photoelastic method a principal stress difference can be measured. Therefore, the sound pressure of the ultrasonic pulse can be measured directly.
The conventional photoelastic method for visually measuring the pulse uses two kinds of approaches. One relies on a linear polariscope and the other on a circular polariscope, in which a stroboscopic light source having a short flash time is used for obtaining a still picture of the pulse travelling with high speed. In the method using the linear polariscope, a high sensitivity sufficient for obtaining a visual image of the pulse can be obtained, but the visual image is undesirably changed in accordance with the direction of the incident polarized light used, because of the linear polarization, therefore a quantitative measurement can not be obtained. On the other hand, in the case of the method using the circular polariscope, a visual image proportional to a principal stress difference can be obtained in principle. Therefore, the quantitative measurement of the pulse waveform and the sound pressure distribution can be obtained. In this case, however, a detailed quantitative measurement can not be achieved because of low sensitivity in visualizing the pulse generated by the conventional ultrasonic flaw detecting apparatus and its probe. Therefore, development of a method for obtaining a visual image of the pulse with high sensitivity by using the photoelastic method has been expected.